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LAB 5: DIFFERENTIAL AMPLIFIER
Title:
Differential Amplifier
Objective:
The differential amplifier is the fundamental building block
of virtually all integrated circuits. The common parameters
desired are the differential gain, common-mode gain, input
offset voltage, common-mode rejection ratio, and commonmode input voltage range. This exercise examines differential
amplifier implemented with BJTs.
Equipment
15 V and -15 V power supply; function generator; 4 discrete
and
npn BJT transistors (2N3904); multi-turn potentiometer (1
components: k); and resistors (values determined in lab).
Note:
Analyze the circuit first, which includes hand calculations or
computer simulation. Compare your experiment measurement
with your analysis predictions. Implement the Widlar current
source from the previous lab to get the desired current output.
Procedure
Build the differential amplifier shown below. Implement your previous Widlar
current source to get the input current (current through R1) of 10 mA and the
collector current of D4 of 1 mA.
a. Ground v+ and v- and adjust the multi-turn potentiometer so that VC1 equal
VC2. Measure IC1, IC2, and VE. Calculate the differential voltage gain using the
equation below and the input offset voltage.
Ad = ( VC1 - VC2 ) / VD = - gmRC.
Also, measure the bias current IB flowing into the base of Q1 and Q2.
Calculate the offset voltage |IB1 - IB2|.
b. Let v+ = 2 mV and v- = -2 mV. Measure VC1, VC2, and VE. Calculate the
differential voltage gain and the single-ended voltage gain at Q2. Repeat this
step for v+ = |v-| = |4 mV|, |6 mV|, |8 mV|, and |10 mV|. (VC1, VC2, and VD are
the small signal changes from their quiescent zero input values from (a)).
From these data, plot the differential voltage gain (VC2 - VC1)/VD vs. VD and
the single-ended differential voltage gain VC2/VD vs. VD. Indicate the range of
VD where the gains are constant.
c. Let v+ = v- = vCM. Vary vCM from + 15 V to - 15 V. Find the range of vCM
where Q1 and Q2 are in the active mode.
d. Let vCM = sin (2 ft) with f = 1 kHz. Measure the common-mode voltage gain
vc2/vCM.
e. Calculate the common-mode rejection ratio (CMRR) in dB.